a = "aBc"
print(string.upper(a)) --> "ABC"

a = "aBc"
print(string.lower(a)) --> "abc"

a = "codergege"
b = "ge"
-- gsub 返回替换后的字符串及替换次数
print(string.gsub(a, b, "bi", 1)) --> "coderbige"   1
-- num 指定为 3 也没关系, 不会报错, 只会替换两次
print(string.gsub(a, b, "bi", 3)) --> "coderbibi"   2 
print(string.gsub(a, b, "bi")) --> "coderbibi"  2
print(string.gsub(a, "hi", "abc")) --> "codergege" 0

a = "codergege%d123"
b = "ge"
print(string.find(a, b)) --> 6  7
print(string.find(a, b, 1)) --> 6   7
print(string.find(a, b, 7 )) --> 8  9
print(string.find(a, "%d", 1, false)) --> 默认开启模式匹配, %d 匹配 1, 所以返回 12  12
print(string.find(a, "%d", 1, true)) --> 关闭模式匹配, %d 匹配 %d, 所以返回 10  11

